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From: adam@irvine.com (Adam Beneschan)
Subject: Re: Uninitialized "out" parameters
Date: 1996/07/19
Message-ID: <4smo24$h25@krusty.irvine.com>#1/1
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distribution: world
references: <4smh9i$gp5@krusty.irvine.com> <4smmhe$9cs@goanna.cs.rmit.edu.au>
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newsgroups: comp.lang.ada
Date: 1996-07-19T00:00:00+00:00
List-Id: <comp.lang.ada>


Dale Stanbrough <dale@goanna.cs.rmit.edu.au> writes:
 
 >Adam Beneschan writes:
 >"Well, you can't read an "out" parameter at all, so it's illegal to use
 > "op" in the right-hand side of your assignment.  So the compiler
 > should give you an error.  Also, in Ada83, you'll get an error because
 > I needs to be declared before the code for SubP appears.
 > 
 > If you change op to an "in out" parameter, you'll be reading an
 > uninitialized variable, but typically neither the compiler nor the
 > runtime will complain."
 > 
 > 
 >Ada95 does allow you to read out parameters. The code written simply
 >uses an uninitialized variable, and I would imagine it would be declared
 >erroneous by the LRM.

Sorry, I wasn't aware of this change.

This rule change frightens me a little.  In the posted example:

   procedure SubP (op : out integer) is
   begin
      op := op + 1;
   end SubP;

I've been known to accidentally type "out" when I mean "in out".  In
Ada 83, the compiler would catch me, but now it won't, and the effect
will be very different because here the "op" on the right side of the
assignment is always uninitialized.  However, I guess a smart compiler
would let me know that I'm using an uninitialized parameter, and
perhaps the advantages of letting you read an OUT parameter after
you've written to it outweigh this disadvantage.

                                -- Adam